Apparatus and method for transmitting/receiving data in a mobile communication system using multiple antennas

ABSTRACT

An apparatus and method for transmitting/receiving data in a mobile communication system using multiple antennas are provided. A receiver estimates a fading channel of received data, selects a weight set relative to a maximum data transmission rate from at least one weight set with elements of a plurality of orthogonal weight vectors, and transmits feedback information including the selected weight set and channel-by-channel state information to a transmitter. The transmitter demultiplexes data to be transmitted on a basis of the feedback information into at least one sub-data stream, multiplies each sub-data stream by an associated weight, and transmits the data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to applicationSer. No. 2005-45834 filed in the Korean Intellectual Property Office onMay 30, 2005, the entire disclosure of which is hereby incorporated byreference. This application is a continuation reissue application ofU.S. patent application Ser. No. 13/871,939, filed on Apr. 26, 2013,which is a reissue application of U.S. Pat. No. 7,933,560, issued onApr. 26, 2011 from U.S. patent application Ser. No. 11/442,317, filed onMay 30, 2006, which claims the benefit under 35 U.S.C. § 119(a) ofKorean Patent Application No. 10-2005-45834, filed on May 30, 2005, theentire content of all of said prior applications and patent being herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an apparatus and method fortransmitting/receiving data in a mobile communication system. Moreparticularly, the present invention relates to a datatransmission/reception apparatus and method for implementing a spatialmultiplexing transmission in a mobile communication system usingmultiple transmit/receive antennas.

2. Description of the Related Art

Mobile communication systems are developing into high-speed,high-quality wireless data packet communication systems for providing adata service and a multimedia service as well as a voice service. Forexample, the standardization for High-Speed Downlink Packet Access(HSDPA) ongoing in the 3rd Generation Partnership Project (3GPP) and thestandardization for 1× Evolution Data and Voice (1×EV-DV) ongoing in the3rd Generation Partnership Project 2 (3GPP2) can be regarded as evidenceof the effort for finding a high-quality wireless data packet transferservice at more than 2 Mbps in the 3G mobile communication system. Onthe other hand, the 4^(th) generation (4G) mobile communication systemserves to provide a higher-speed, higher-quality multimedia service.

To provide a high-speed, high-quality data service in wirelesscommunication, a spatial multiplexing transmission scheme has beenproposed which exploits a Multiple-Input Multiple-Output (MIMO) antennasystem with multiple antennas in transmitting and receiving stages. Thespatial multiplexing transmission scheme simultaneously transmitsdifferent data streams on a transmit antenna-by-transmit antenna basis.It is known that the possible service data capacity linearly increasesin proportion to the number of transmit/receive antennas as the numberof transmit/receive antennas increases without an increase in anadditional frequency bandwidth theoretically.

When fading between the transmit/receive antennas is independent, thespatial multiplexing transmission scheme provides high capacity inproportion to the number of transmit/receive antennas. The capacity issignificantly reduced in an environment with a high spatial correlationof fading rather than an independent fading environment. This is becausefading from which signals transmitted from the transmit antennas sufferis similar and therefore it is difficult for the receiving stage tospatially distinguish a signal. Possible transmission capacity isaffected by a Signal to Noise Ratio (SNR). As the received SNRdecreases, the transmission capacity decreases. Thus, a transmissiondata rate can be maximized when the number of data streams to besimultaneously transmitted and a transmission rate of each data streamare adjusted according to radio channel states, in other words a spatialcorrelation of fading and a received SNR. If a transmission rate of datato be transmitted exceeds the transmission capacity supportable by aradio channel, many errors occur due to interference between datastreams to be simultaneously transmitted and an actual data transmissionrate decreases.

To increase a transmission data rate in the spatial multiplexingtransmission scheme, profound research on a precoding scheme has beenconducted. The precoding scheme multiplies data streams to betransmitted from a transmitter by transmission weights and transmits thedata streams using information about a downlink channel from thetransmitter to a receiver. Thus, the transmitter is to know a state of adownlink channel from each transmit antenna of the transmitter to eachreceive antenna of the receiver. For this, the receiver is to estimatethe downlink channel state and feed back information about the downlinkchannel state estimated through a feedback channel. However, thereceiver is to transmit a large quantity of feedback data using anuplink feedback channel in order to feed back the downlink channel stateinformation. When a large amount of feedback data is to be transmitted,much time is taken to feed back the downlink channel state informationfrom the receiver to the transmitter using the uplink feedback channelwhose bandwidth is limited. The conventional precoding scheme cannot beapplied to an instantaneously varying wireless channel environment.Accordingly, a real need exists for technology for maximizing a datatransmission rate by precoding while minimizing an amount of feedbackdata needed to be transmitted from the receiver to the transmitter.

A precoder codebook scheme has been proposed as the conventionaltechnology for reducing an amount of feedback information. In theprecoder codebook scheme, the receiver selects a precode with themaximum transmission rate from among candidate precodes of a precodercodebook constructed by a limited number of precodes known to thetransmitter and the receiver, and feeds back an index of the selectedprecode to the transmitter. The transmitter sends data using a precodemapped to the fed-back index in the precoder codebook. For example, when4-bit feedback information is used, a precoder codebook constructed by amaximum of 2⁴ (=16) precodes is preset between the transmitter and thereceiver. Because fading varies with time, the precode decision processis to be repeated in every time slot, such that the selected precodeindex is fed back to the transmitter in every time slot.

In comparison with the precoding scheme using the feedback channel stateinformation, the precoder codebook scheme requires a smaller amount offeedback information. Assuming that the number of transmit antennas andthe number of receive antennas are n_(T) and n_(R) in the MIMO antennasystem, respectively, a total of n_(T)×n_(R) complex channelcoefficients must be fed back when the channel state information is fedback. When Q bits are required to indicate one complex channelcoefficient, a total of n_(T)×n_(R)×Q bits are required. On the otherhand, the precoder codebook scheme requires ┌log₂K┐ bits when the numberof precodes for providing a sufficient data rate is K, where ┌x┐ is aninteger equal to or more than x. In the precoding scheme using thechannel state information, an amount of feedback information increasesin proportion to a product of the number of transmit antennas and thenumber of receive antennas. However, in the precoder codebook scheme, anamount of feedback information depends on the number of precodescontained in the precoder codebook, in other words a size of theprecoder codebook.

The precoder codebook scheme is to include, in the codebook, ready-madeprecodes quantized in all possible cases at a spatial multiplexingtransmission time. The precoder codebook scheme can reduce an amount offeedback information using predefined precodes, and can also reduce thedegree of freedom in a precoding matrix. When the number of factors tobe considered is large, the degree of freedom in the preceding matrixsignificantly increases the number of preset precodes, such that a sizeof the precoder codebook increases. In the following two cases, a sizeof the precoder codebook significantly increases.

First, the number of precodes to be considered increases at the ratio ofgeometrical progression because all precodes are to be consideredaccording to a spatial correlation of a channel for an application in achannel environment with various spatial correlations. An optimalprecoder codebook differs according to a spatial correlation of achannel. In the conventional precoder codebook technology, the precodercodebook is designed under the assumption that a fading channel does nothave a spatial correlation. However, a distribution of valideigenvalues, in other words eigenvectors whose eigenvalues are large,differs, and therefore optimal precodes differ, according to the spatialcorrelation of the fading channel. As a result, a large number ofprecoder codebooks optimized according to the spatial correlation of thefading channel are to be used to achieve a high data transmission rate.

Second, the number of precodes to be considered increases at the ratioof geometrical progression because all precodes are to be consideredaccording to the number of data streams to be simultaneously transmittedwhen the number of data streams to be simultaneously transmitted isadjusted according to a channel environment. The number of data streamsto be simultaneously transmitted varies from 1 to a maximum ofmin(n_(T),n_(R)) (indicative of a minimum value between the number oftransmit antennas and the number of receive antennas). The number ofcolumns of a precode matrix is to be varied according to the number ofdata streams to be simultaneously transmitted. Because column vectorsfor constructing the precode matrix serving as weight vectors aremultiplied by data streams, the number of column vectors of the precodematrix is to match the number of data streams to be simultaneouslytransmitted. For example, when both the number of transmit antennas andthe number of receive antennas are 4, the number of data streams capableof being simultaneously transmitted varies from 1 to 4. There must beconsidered precodes in which the number of column vectors is 1, precodesin which the number of column vectors is 2, precodes in which the numberof column vectors is 3, and precodes in which the number of columnvectors is 4. When the maximum number of data streams capable of beingsimultaneously transmitted increases as the number of transmit/receiveantennas increases, a significantly increased amount of feedbackinformation is required according to an increased number of precodes tobe considered. Thus, it is difficult for the precoder codebook scheme tobe applied to the spatial multiplexing transmission scheme for obtainingthe maximum transmission rate in an associated channel environment byvarying a transmission data rate and the number of data streams to besimultaneously transmitted according to the channel environment. In theprecoder codebook scheme using a set of predefined precodes as describedabove, a size of a precoder codebook increases according to the numberof transmit antennas and the number of data streams to be simultaneouslytransmitted, such that its actual application may be difficult.

The number of antennas may be different between receivers communicatingwith one transmitter. For example, when the number of base stationantennas is 4 and the number of mobile station antennas is 1, 2, 3, or 4according to a terminal type, the maximum number of sub-data streamscapable of being transmitted becomes 1, 2, 3, or 4. When the precodercodebook technology is applied, each precoder codebook based on thenumber of all available receiver antennas and a feedback channel basedon each precoder codebook must be defined. The receivers are to selectand use a precoder codebook based on the number of antennas of anassociated receiver and a feedback channel based on the precodercodebook. A process for defining a precoder codebook and feedbackinformation to be used between a transmitter and a receiver is required.Thus, a flexible precoding scheme applicable to various transmit/receiveantenna structures is required.

Accordingly, there is a need for an improved and efficient precodingscheme and a feedback scheme that can be applied to a spatialmultiplexing transmission scheme for adjusting the number of datastreams to be simultaneously transmitted in a channel environment withvarious spatial correlations and can provide a high data transmissionrate with a significantly small amount of feedback information.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention address at least theabove problems and/or disadvantages and provide at least the advantagesdescribed below. Accordingly, it is, therefore, an object of the presentinvention to provide an apparatus and method for transmitting/receivingdata that can efficiently provide a data transmission rate according toa channel environment in a mobile communication system using multipletransmit/receive antennas.

It is another object of the present invention to provide an apparatusand method for transmitting/receiving data that can provide a high datatransmission rate with a small amount of feedback information in amobile communication system using multiple transmit/receive antennas.

In accordance with an exemplary aspect of the present invention, thereis provided a mobile communication system using multiple antennas,comprising a receiver for estimating a fading channel of received data,selecting a weight set relative to a maximum data transmission rate fromat least one weight set with elements of a plurality of orthogonalweight vectors, and transmitting feedback information including theselected weight set and channel-by-channel state information to atransmitter, and the transmitter for demultiplexing data to betransmitted on a basis of the feedback information into at least onesub-data stream, multiplying each sub-data stream by an associatedweight, and transmitting the data.

An exemplary receiver may comprise a downlink channel estimator forestimating a channel state using a pilot channel of the data transmittedfrom the transmitter, a weight selector for deciding the weight set andweight vectors on a basis of the channel state, and transmittinginformation about the weight set and the weight vectors to thetransmitter, and a subchannel-by-subchannel state estimator forestimating channel states of sub-data streams according to the decidedweight vectors and transmitting only information about the channelstates of the sub-data streams to the transmitter.

An exemplary receiver may comprise a downlink channel estimator forestimating a channel state using a pilot channel of the data transmittedfrom the transmitter; a weight selector for deciding-the weight set andweight vectors on a basis of the channel state, and transmitting thedecided weight set and the decided weight vectors to the transmitter anda subchannel-by-subchannel state estimator for estimating channel statesof all weight vectors of the decided weight set and transmittinginformation about the estimated channel states to the transmitter.

An exemplary subchannel-by-subchannel state estimator transmitsinformation about a “No Transmission” state for an unused channel on abasis of the decided weight vectors.

An exemplary transmitter comprises a demultiplexer for demultiplexing amain data stream to be transmitted into the at least one sub-datastream, at least one channel encoder and modulator for receiving the atleast one sub-data stream and independently performing channel codingand modulation processes for the at least one sub-data stream accordingto a channel coding rate and a modulation scheme, a beamformer formultiplying the at least one channel-coded and modulated sub-data streamby a weight and transmitting the data to the receiver, and a controllerfor deciding in advance the number of sub-data streams, the coding rateof the at least one sub-data stream, the modulation scheme, and a weightto be multiplied by each sub-data stream on a basis of the feedbackinformation transmitted from the receiver.

The feedback information may comprise weight set index information forindicating the selected weight set, weight vector information forindicating weight vectors selected from the selected weight set, andchannel state information of the at least one sub-data stream.

The transmitter and the receiver may store in advance weight sets andweight vectors according to the number of transmit antennas, the numberof receive antennas, and the number of weight sets.

In accordance with another exemplary aspect of the present invention,there is provided a method for transmitting/receiving data in a mobilecommunication system using multiple antennas, comprising a) estimating afading channel from a pilot channel of received data in a receiver, b)selecting a weight set relative to a maximum data transmission rate fromat least one weight set with elements of a plurality of orthogonalweight vectors on a basis of the estimated fading channel, c) estimatingchannel-by-channel state information relative to the selected weightset, d) transmitting feedback information comprising the selected weightset and the channel-by-channel state information to a transmitter, ande) transmitting antenna-by-antenna data to be transmitted on a basis ofthe feedback information.

The designing the weight set may comprise deciding a plurality of weightvectors with a phase difference defined according to the number oftransmit antennas and the number of weight sets and configuring theweight set with orthogonal weight vectors among the decided weightvectors.

The phase difference may be computed by

$\frac{2\pi}{N \cdot n_{T}},$where N is the number of weight sets and n_(T) is the number of transmitantennas.

The designing the weight set may comprise f-1) deciding a plurality oforthogonal weight vectors among a number of weight sets, and f-2)repeating f-1) a number of times corresponding to the number of weightsets.

The f-1) may comprise deciding a reference phase for orthogonal elementsof a decided weight vector, and deciding elements with a difference ofthe reference phase from a first element of the weight vector.

The feedback information may comprise weight set index information forindicating the selected weight set, weight vector information forindicating a weight vectors selected from the selected weight set, andchannel state information of at least one sub-data stream.

The feedback information may comprise weight set index information forindicating the selected weight set, and channel state information of allsub-data streams relative to the selected weight set.

The e) may comprise: demultiplexing a main data stream to be transmittedinto at least one sub-data stream on the basis of the feedbackinformation, independently performing channel coding and modulationprocesses for the at least one sub-data stream according to a channelcoding rate and a modulation scheme defined on the basis of the feedbackinformation, and multiplying the at least one channel-coded andmodulated sub-data stream by a weight defined on the basis of thefeedback information and transmitting the data to the receiver.

In accordance with another exemplary aspect of the present invention,there is provided a mobile communication system using multiple antennas,comprising a receiver for estimating a fading channel of received data,applying at least one weight set with elements of a plurality oforthogonal weight vectors in a time period, deciding weight vectorsrelative to a maximum data transmission rate for the at least one weightset to be used at a time point, and transmitting feedback informationcomprising channel-by-channel state information and the decided weightvectors to a transmitter, and the transmitter for receiving the feedbackinformation, demultiplexing data, to be transmitted on a basis of theweight vectors of the weight set to be applied in the time period, intoat least one sub-data stream, multiplying the at least one sub-datastream by an associated weight, and transmitting the data.

The receiver comprises a downlink channel estimator for estimating achannel state using a pilot channel of the data transmitted from thetransmitter, a weight selector for deciding information about the weightvectors of the weight set to be applied in the time period on a basis ofthe channel state and transmitting the decided weight vector informationto the transmitter, and a subchannel-by-subchannel state estimator forestimating channel states of the decided weight vectors and transmittinginformation about the estimated channel states to the transmitter.

The transmitter comprises a demultiplexer for demultiplexing a main datastream to be transmitted into at least one sub-data stream, at least onechannel encoder and modulator for receiving the at least one sub-datastream and independently performing channel coding and modulationprocesses for the at least one sub-data stream according to a channelcoding rate and a modulation scheme, a beamformer for multiplying the atleast one channel-coded and modulated sub-data stream by a weight andtransmitting the data to the receiver, and a controller for deciding inadvance the number of sub-data streams, the coding rate of the at leastone sub-data stream, the modulation scheme, and a weight to bemultiplied by each sub-data stream on a basis of the feedbackinformation transmitted from the receiver when the weight set isapplied.

The transmitter and the receiver may store in advance weight sets andweight vectors according to the number of transmit antennas, the numberof receive antennas, and the number of weight sets.

In accordance with yet another exemplary aspect of the presentinvention, there is provided a method for transmitting/receiving data ina mobile communication system using multiple antennas, the methodcomprising estimating a fading channel from a pilot channel of receiveddata in a receiver, applying at least one weight set with elements of aplurality of orthogonal weight vectors in a time period on a basis ofthe estimated fading channel and deciding weight vectors relative to amaximum data transmission rate for the at least one weight set to beused at a time point, estimating channel-by-channel state informationrelative to the decided weight vectors, transmitting feedbackinformation comprising the decided weight vectors and thechannel-by-channel state information to a transmitter, and receiving thefeedback information and transmitting antenna-by-antenna data accordingto the weight vectors of the weight set to be applied in the timeperiod.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and aspects of the present invention will bemore clearly understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates a system in accordance with a first exemplaryembodiment of the present invention;

FIG. 2 is a flowchart illustrating a method for transmitting/receivingdata in a receiver of the system in accordance with the first exemplaryembodiment of the present invention;

FIG. 3 is a flowchart illustrating a method for transmitting/receivingdata in a transmitter of the system in accordance with the firstexemplary embodiment of the present invention;

FIGS. 4 and 5 are flowcharts illustrating a method for deciding a weightset in the system of the present invention;

FIG. 6 illustrates a system in accordance with a second exemplaryembodiment of the present invention;

FIG. 7 is a flowchart illustrating a method for transmitting/receivingdata in a receiver of the system in accordance with the second exemplaryembodiment of the present invention;

FIG. 8 is a flowchart illustrating a method for transmitting/receivingdata in a transmitter of the system in accordance with the secondexemplary embodiment of the present invention;

FIG. 9 illustrates a system in accordance with a third exemplaryembodiment of the present invention;

FIG. 10 is a flowchart illustrating a method for transmitting/receivingdata in a receiver of the system in accordance with the third exemplaryembodiment of the present invention;

FIG. 11 is a flowchart illustrating a method for transmitting/receivingdata in a transmitter of the system in accordance with the thirdexemplary embodiment of the present invention;

FIG. 12 is a graph illustrating simulation results of a systemperformance comparison between the conventional technology and theproposed system in an environment in which a spatial correlation ispresent; and

FIG. 13 is a graph illustrating simulation results of a systemperformance comparison between the conventional technology and theproposed system in an environment in which a spatial correlation isabsent.

Throughout the drawings, the same drawing reference numerals will beunderstood to refer to the same elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be described indetail herein below with reference to the accompanying drawings. In thefollowing description, details are provided for a better understandingof the present invention. In the following description, detaileddescriptions of functions and configurations incorporated herein thatare well known to those skilled in the art are omitted for clarity andconciseness.

The present invention proposes an apparatus and method in which atransmitter receives and efficiently uses a receiver's feedbackinformation according to a spatial correlation in a system with multipletransmit/receive antennas.

In the system using the multiple transmit/receive antennas in theexemplary embodiments of the present invention, the receiver selects aweight set relative to a maximum data transmission rate from a pluralityof weight sets, selects weights of the weight set, and transmitsselection information through an uplink feedback channel to thetransmitter. The transmitter constructs a preceding matrix using theinformation transmitted through the feedback channel from the receiver.Herein, the information may be an index of the weight set, weight vectorinformation corresponding to information about weights selected from theweight set, and channel state information of respective sub-datastreams. In the exemplary embodiments of the present invention, theinformation including the index of the weight set, the weight vectorinformation corresponding to the information about the weights selectedfrom the weight set, and the channel state information of the respectivesub-data streams is defined as the feedback information. The technologyproposed in the present invention is referred to as the knockdownpreceding technology.

Next, a system and feedback information generation method in accordancewith the present invention will be described with reference to theexemplary embodiments.

1. First Exemplary Embodiment

1) Knockdown Precoding System

The present invention considers a system of multiple transmit/receiveantennas in which a transmitter has a transmit antenna array in whichn_(T) antennas are arrayed and a receiver has a receive antenna array inwhich n_(R) antennas are arrayed. Multiple weight sets are predefinedbetween the transmitter and the receiver. The weight set is a set ofweight vector elements whose number corresponds to the number oftransmit antennas. When N weight sets are decided, a total of N×n_(T)weight vectors are decided.

In the knockdown preceding technology, the receiver selects a weight setrelative to a maximum data transmission rate from a plurality of weightsets, selects weights from the weight set, and transmits selectioninformation through an uplink feedback channel to the transmitter. Thetransmitter constructs a precoding matrix using the transmittedinformation.

FIG. 1 illustrates a system in accordance with a first exemplaryembodiment of the present invention. For convenience of explanation, thefirst exemplary embodiment of the present invention corresponds to thecase where the number of transmitter antennas and the number of receiverantennas are 2, respectively.

Referring to FIG. 1, a receiver 130 of a system 100 of the presentinvention is provided with a downlink (DL) channel estimator 133, ademodulator 131, a weight selector 135, a subchannel-by-subchannel stateestimator 137, and a multiplexer (MUX) 139 according to functions. Atransmitter 110 is provided with a controller 111, a demultiplexer(DEMUX) 113, channel encoders/modulators 115 and 117, and beamformers119 and 121.

The downlink channel estimator 133 performs channel estimation using apilot channel of a signal received from the transmitter 110 andtransfers estimation information to the weight selector 135. The weightselector 135 generates weight sets and weight vectors of each weight setaccording to the number of antennas on the basis of the estimationinformation. The weight selector 135 transmits a weight set index 151and weight vector information 153 to the transmitter 110.Simultaneously, the weight selector 135 transfers the information to thesubchannel-by-subchannel state estimator 137. Thesubchannel-by-subchannel state estimator 137 estimateschannel-by-channel states relative to a weight set selected according tothe received information and transmits channel-by-channel stateinformation to the transmitter 110.

The controller 111 of the transmitter 110 receives feedback information150 from the receiver 130. The controller 111 controls the demultiplexer113, the channel encoders 115 and 117, and the beamformers 119 and 121using the feedback information 150. In detail, the controller 111decides the number of sub-data streams using the feedback information150 and notifies the demultiplexer 113 of the decided number of sub-datastreams. The controller 111 decides a coding rate and a modulationscheme of each sub-data stream on the basis of channel state information155 of each sub-data stream among feedback information 150 and notifiesthe channel encoders/modulators 115 and 117 of the decided coding rateand the decided modulation scheme. At a beamforming time, the controller111 computes weights to be applied to the respective sub-data streamsusing a weight set index 151 or information 153 of weights selected froman associated weight set among the feedback information 150, andnotifies the beamformers 119 and 121 of the computed weights.

The demultiplexer 113 demultiplexes a main data stream according to thenumber of sub-data streams transmitted from the controller 111. Thechannel encoders/modulators 115 and 117 independently encode andmodulate sub-data streams obtained by demultiplexing the main datastream using information about the coding rate and the modulation schemereceived from the controller 111. The beamformers 119 and 121 multiplythe sub-data streams received from the channel encoders/modulators 115and 117 by weights. The transmitter 110 computes a sum of the sub-datastreams and transmits data through transmit antennas 123.

A method for transmitting data in a transmitter and a receiver of thesystem of the present invention will be described in detail withreference to FIGS. 2 and 3.

FIG. 2 is a flowchart illustrating an exemplary method fortransmitting/receiving data in the receiver 130 of the system of FIG. 1.

Referring to FIG. 2, the downlink channel estimator 133 of the receiver130 estimates a downlink fading channel using a pilot channel or symbolreceived from multiple receive antennas 139 in step 201. That is, thedownlink fading channel from each transmit antenna to each receiveantenna is estimated. Subsequently, the weight selector 135 selectsweight information relative to a maximum data transmission rate on thebasis of information about the estimated fading channel in step 203.Herein, the weight information is a weight set index 151 and weightvector information 153.

Step 203 will be described in detail. Weight vectors relative to themaximum data transmission rate are selected from each of the N weightsets, and a possible transmission data rate based on the selected weightvectors is computed. That is, possible transmission data rates arecompared between the N selected weight sets (or the weight sets withelements of weight vectors selected from the respective weight sets),and a weight set with the maximum data transmission rate is selected. Aweight set index belonging to the weight set with the maximumtransmission rate is decided and weights to be used to actually transmitweight vectors belonging to the weight set relative to the maximumtransmission rate are decided.

The subchannel-by-subchannel state estimator 137 estimates channels ofrespective sub-data streams according to the weight information in step205. That is, Signal to Interference Noise Ratios (SINRs) of thesub-data streams formed by the weights selected by the weight selector135 are computed, and Modulation and Coding Selection (MCS) or channelstate information of each sub-data stream is decided. Subsequently, thereceiver 130 transmits the feedback information 150 including the weightinformation and the channel state information to the transmitter in step207. Herein, the receiver can simultaneously or separately transmitelements of the feedback information.

FIG. 3 is a flowchart illustrating an exemplary method fortransmitting/receiving data in the transmitter 110 of the system of FIG.1.

Referring to FIG. 3, the controller 111 of the transmitter 110 receivesfeedback information 150 from the receiver 130 in step 301.Subsequently, the controller 111 decides the number of sub-data streamscapable of being finally transmitted using weight information of thefeedback information 150 in step 303. Herein, the number of sub-datastreams capable of being transmitted is equal to the number of selectedweights.

The demultiplexer 113 demultiplexes a main data stream to be transmittedinto sub-data streams whose number corresponds to the number of sub-datastreams capable of being transmitted in step 305. Using informationabout a coding rate and a modulation scheme defined by the fed-backchannel state information of the respective sub-data streams, thechannel encoders/modulators 115 and 117 independently perform encodingand symbol mapping processes for the sub-data streams in step 307.Subsequently, the beamformers 119 and 121 multiply the sub-data streamsby weights transferred from the controller 111 and transmit the sub-datastreams through the transmit antennas 123 in step 309.

To feed back a precode constructed by weights relative to a maximum datatransmission rate to the transmitter 110 in a process for obtaining aweight set and its weight vectors in an exemplary embodiment of thepresent invention, a feedback channel is required to transmit a selectedweight set index 151 and weight vector information 153 about the weightsselected from the selected weight set. N weight sets are designed byEquation (1). If the N weight sets are defined by the transmitter andreceivers within a cell, the number of bits allocated to the feedbackchannel for feeding back the selected weight set index 151 is └log₂N┘.Herein, └x┘ is a minimum integer equal to or more than x.

When a scheme for indicating selection or non-selection of each weightbelonging to a selected weight set is used for the weights selected fromone weight set, 1-bit feedback information for each weight is required,such that feedback bits whose number corresponds to the total number oftransmit antennas are required. An amount of feedback informationrequired to feed back a precode is └log₂N┘+n_(T) bits/use. A feedbackchannel is additionally required to feed back channel state informationof respective sub-data streams formed by weights estimated and selectedby the subchannel-by-subchannel state estimator 137.

Next, a method for designing a weight set in accordance with anexemplary embodiment of the present invention will be described.

2) Weight Set Design for Knockdown Precoding Technology

The transmitter 110 and the receiver 130 define multiple weight sets.The weight set is a set of elements of weight vectors whose numbercorresponds to the number of transmit antennas, n_(T). The weight vectormay be referred to as the weight. Herein, one weight vector isconstructed by n_(T) complex elements. When N weight sets are defined, atotal of (N×n_(T)) weight vectors are constructed. When the N weightsets are designed, the following principles are used to consider aspatial correlation. First, n_(T) weights belonging to one weight setare orthogonal to each other, and the magnitude of each weight is 1.

Second, main beam directions of beams formed by the total of (N×n_(T))weight vectors do not overlap with each other and are to be uniformlydistributed within a service area.

To decide the total of N weight sets satisfying the first and secondprinciples, the total of (N×n_(T)) weight vectors in which a phasedifference between elements neighboring to each weight vector is aninteger multiple of

$\frac{2\pi}{N \cdot n_{T}}$are generated, the n_(T) weights in which a phase difference betweenweight vector elements in the same position is an integer multiple of

$\frac{2\pi}{n_{T}}$are grouped in one weight set, and the total of N weight sets in whichthe n_(T) weights belonging to the same weight set are orthogonal toeach other are decided.

FIG. 4 illustrates an example of a process for deciding the total of Nweight sets.

Referring to FIG. 4, (N×n_(T)) weight vectors are generated in step 400.The number of weight sets, N, and the number of transmit antennas,n_(T), are input. To compute the (N×n_(T)) weight vectors, a cyclicprocess of steps 401˜405 from k=0 to k=(N×n_(T)) is performed. In step402, a phase difference

$\left( {\Delta_{k} = \frac{2{\pi k}}{{Nn}_{T}}} \right)$between neighbor elements within a weight vector is computed in order tocompute the k-th weight vector. In step 403, the k-th weight vector isdecided using the computed phase difference. The first element of thek-th weight vector is

$\frac{1}{\sqrt{n_{T}}},$the second element is

$\frac{1}{\sqrt{n_{T}}}{\exp\left( {j\;\Delta_{k}} \right)}$with a phase of Δ_(k), inother words

${\frac{1}{\sqrt{n_{T}}}{\exp\left( {j\frac{2{\pi k}}{{Nn}_{T}}} \right)}},$and the third element is

$\frac{1}{\sqrt{n_{T}}}{\exp\left( {j\; 2\Delta_{k}} \right)}$with a phase that is Δ_(k) more than that of the second element, inother words

$\frac{1}{\sqrt{n_{T}}}{{\exp\left( {j\frac{4{\pi k}}{{Nn}_{T}}} \right)}.}$When all the n_(T) elements are filled in the above-described method,the k-th weight vector is completed. After the k-th weight vector isdecided, k is incremented by 1 in step 404. When steps 402 and 403 arerepeated, the (k+1)-th weight vector is decided. After all the (N×n_(T))weight vectors are decided in step 406, only orthogonal weight vectorsare collected and are classified into the weight sets in step 407. Aclassification criterion is to collect n_(T) weights in which a phasedifference between weight vector elements in the same position is aninteger multiple of

$\frac{2\pi}{n_{T}}$in one weight set. When the weight sets are classified such that thecriterion is satisfied, Weight Set 1 is constructed by k-th weightvectors where k=0, N, 2N, . . . , (n_(T)−1)N, and Weight Set 2 isconstructed by k-th weight vectors where k=1, N+1, 2N+1, . . . ,(n_(T)−1)N+1. In general expression, Weight Set (n+1) is constructed byk-th weight vectors where k=n, N+n, 2N+n, . . . , (n_(T)−1)N+n.

An example of a concrete design according to the above-describedprinciples for designing weight sets is as follows. When N weight sets{E_(n} (n=)1, L, N) are designed, each weight set E_(n) is constructedby elements of n_(T) orthogonal weight vectors {e_(n,i)} (n=1, L,n_(T)). That is, E_(n)={e_(n.1), e_(n.2), L, e_(n.nT)}. Herein,{e_(n,i)} denotes the i-th weight vector belonging to the n-th weightset E_(n) and is designed as shown in Equation (1).

$\begin{matrix}{e_{n,\; i} = {{\frac{1}{\sqrt{n_{T}}}\begin{bmatrix}\omega_{1,\; i}^{(n)} \\\vdots \\\omega_{n_{T},\; i}^{(n)}\end{bmatrix}} = {\frac{1}{\sqrt{n_{T}}}\begin{bmatrix}1 \\e^{j\;\frac{\;{2\;\pi}}{n_{T}}\;{({\frac{n - 1}{N} + {({i - 1})}})}} \\e^{j\; 2\;\frac{\;{2\;\pi}}{n_{T}}\;{({\frac{n - 1}{N} + {({i - 1})}})}} \\\vdots \\e^{{j{({n_{T}\; - \; 1})}}\;\frac{2\;\pi}{n_{T}}\;{({\frac{n - 1}{N} + {({i - 1})}})}}\end{bmatrix}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

Herein, ω_(m,i) ^((n)) is expressed by Equation (2).

$\begin{matrix}{\omega_{m,\; i}^{(n)} = \;{{\exp\;\left\{ {{j\left( {m - 1} \right)}\;\phi_{n,\; i}} \right\}} = {\exp\;\left\{ {j\;\frac{\;{2{\pi\left( {m - 1} \right)}}}{n_{T}}\;\left( {\frac{n - 1}{N} + i - 1} \right)} \right\}}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

Herein,

$\phi_{n,i} = \left( {\frac{2\pi}{n_{T}}\left( {\frac{n - 1}{N} + i - 1} \right)} \right)$denotes a reference phase of the i-th weight vector belonging to then-th weight set E_(n).

FIG. 5 is a flowchart illustrating another example of deciding a weightset in accordance with an exemplary embodiment of the present invention.Specifically, FIG. 5 illustrates a process for deciding a total of Nweight sets according to Equation (1). First, a weight set number n isinitialized to 1 in step 500. Because the n-th weight set is computed instep 501, the first weight set is computed immediately after step 500.In step 502, n is incremented by 1. Until all the N weight sets arecompleted, step 501 is repeated. When all the weight sets are completed,the process is ended in step 504.

In step 501, n_(T)weight vectors within the n-th weight set arecomputed. In step 510, a weight vector number i of the n-th weight setis initialized to 1. In step 511, the i-th weight vector of the n-thweight set is decided. That is, the first weight vector of the n-thweight set is computed after step 510. In step 512, i is incrementedby 1. Until a total of n_(T) weight vectors within the n-th weight setare completed, step 511 is repeated. When all weight vectors of the n-thweight set are decided, the n-th weight set is completely decided instep 514. Then, a process for deciding the next weight set is performed.

In step 511, a process for computing the i-th weight vector of the n-thweight set is performed. In step 520, a reference phase ϕ_(n.i) isdecided to compute the i-th weight vector of the n-th weight set. Whenthe reference phase is decided, each element of the i-th weight vectorwithin the n-th weight set is computed with a value of the referencephase. In step 521, an element number m is initialized to 1. In step522, the m-th element of the i-th weight vector within the n-th weightset is computed by applying the reference phase computed in step 520 toω_(m,i) ^((n))=exp{j(m−1)ϕ_(n,i)}. That is, the first element of thei-th weight vector within the n-th weight set is computed immediatelyafter step 521. When this process is repeated from m=1 to m=n_(T), thei-th weight vector within the n-th weight set is completed in step 525.Then, a process for deciding the next weight vector is performed.

When two weight sets are designed in a system of multipletransmit/receive antennas including four transmit antennas, they areexpressed as shown in Equation (3).

$\begin{matrix}{{E_{1} = {\left\{ {e_{1,\; 1},e_{1,\; 2},e_{1,\; 3},e_{1,\; 4}} \right\} = \left\{ {{\frac{1}{2}\begin{bmatrix}1 \\1 \\1 \\1\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 \\e^{{j\pi}/2} \\e^{j\pi} \\e^{{j{3\pi}}/2}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 \\e^{j\pi} \\e^{j{2\pi}} \\e^{j{3\pi}}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 \\e^{{j{3\pi}}/2} \\e^{j{3\pi}} \\e^{{j{9\pi}}/2}\end{bmatrix}}} \right\}}}{E_{2} = {\left\{ {e_{2,\; 1},e_{2,\; 2},e_{2,\; 3},e_{2,\; 4}} \right\} = \left\{ {{\frac{1}{2}\begin{bmatrix}1 \\e^{{j\pi}/4} \\e^{{j\pi}/2} \\e^{{j{3\pi}}/4}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 \\e^{{j{3\pi}}/4} \\e^{{j{3\pi}}/2} \\e^{{j{9\pi}}/4}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 \\e^{{j{5\pi}}/4} \\e^{{j{5\pi}}/2} \\e^{{j{15\pi}}/4}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}1 \\e^{{j{7\pi}}/4} \\e^{{j{7\pi}}/2} \\e^{{j{21\pi}}/4}\end{bmatrix}}} \right\}}}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$

Four weights belonging to E₁ of Equation (3) are orthogonal to eachother and their magnitudes are 1. Also, four weights belonging to E₂ ofEquation (3) are orthogonal to each other and their magnitudes are 1.However, {e_(1,i)} (i=1, 2, 3, 4) and {e_(2,i)} (i=1, 2, 3, 4) weightsbelonging to different weight sets are not orthogonal to each other.When data streams based on the orthogonal weights are transmitted,interference between the data streams to be simultaneously transmittedis minimized, such that a sum of transmission rates of the data streamsto be simultaneously transmitted can be maximized. The exemplaryknockdown preceding technology proposed in the present invention designsorthogonal weights belonging to one weight set and transmits datastreams to be simultaneously transmitted according to weights selectedfrom the one weight set, thereby reducing interference between the datastreams to be simultaneously transmitted and maximizing a sum oftransmission rates of the data streams to be simultaneously transmitted.Main beams/lobes formed by 8 weights belonging to E₁ and E₂ do notoverlap with each other and are uniformly distributed within a servicearea. Accordingly, even though receivers randomly distributed in theservice area of the transmitter are located in any direction,beamforming gain is obtained by one or more weights of the 8transmission weights.

When weights are selected among the total of (N×n_(T)) weights such thata sum of transmission rates of sub-data streams to be simultaneouslytransmitted is maximized, a probability in which the selected weightsbelong to the same weight set is high. When a hierarchical expressionscheme is used to select one weight set and express weights selectedfrom an associated weight set, an amount of feedback information forexpressing the selected weights in which a data transmission rate ismaximized can be minimized.

Examples of satisfying Equation (1) with respect to the number oftransmit antennas, n_(T), and N weight sets in the exemplary system ofthe present invention are shown in Tables 1 to 12. In the followingtables, (x, y) denotes a complex number in which a real component is xand an imaginary component is y. That is, (x, y)=x+yi.

TABLE 1 Number of Transmit Antennas [n_(T)]: 2, Number of Weight Sets[N]: 1 Set Weight 1 Weight 2 1 (0.7071, 0.0000) (0.7071, 0.0000)(0.7071, 0.0000) (−0.7071, 0.0000)

TABLE 2 Number of Transmit Antennas [n_(T)]: 2, Number of Weight Sets[N]: 2 Set Weight 1 Weight 2 1 (0.7071, 0.0000) (0.7071, 0.0000)(0.7071, 0.0000) (−0.7071, 0.0000) 2 (0.7071, 0.0000) (0.7071, 0.0000)(0.0000, 0.7071) (0.0000, −0.7071)

TABLE 3 Number of Transmit Antennas [n_(T)]: 2, Number of Weight Sets[N]: 3 Set Weight 1 Weight 2 1 (0.7071, 0.0000) (0.7071, 0.0000)(0.7071, 0.0000) (−0.7071, 0.0000) 2 (0.7071, 0.0000) (0.7071, 0.0000)(0.3536, 0.6124) (−0.3536, −0.6124) 3 (0.7071, 0.0000) (0.7071, 0.0000)(−0.3536, 0.6124) (0.3536, −0.6124)

TABLE 4 Number of Transmit Antennas [n_(T)]: 2, Number of Weight Sets[N]: 4 Set Weight 1 Weight 2 1 (0.7071, 0.0000) (0.7071, 0.0000)(0.7071, 0.0000) (−0.7071, 0.0000) 2 (0.7071, 0.0000) (0.7071, 0.0000)(0.5000, 0.5000) (−0.5000, −0.5000) 3 (0.7071, 0.0000) (0.7071, 0.0000)(0.0000, 0.7071) (0.0000, −0.7071) 4 (0.7071, 0.0000) (0.7071, 0.0000)(−0.5000, 0.5000) (0.5000, −0.5000)

TABLE 5 Number of Transmit Antennas [n_(T)]: 3, Number of Weight Sets[N]: 1 Set Weight 1 Weight 2 Weight 3 1 (0.5774, 0.0000) (0.5774,0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (−0.2887, 0.5000) (−0.2887,−0.5000) (0.5774, 0.0000) (−0.2887, −0.5000) (−0.2887, 0.5000)

TABLE 6 Number of Transmit Antennas [n_(T)]: 3, Number of Weight Sets[N]: 2 Set Weight 1 Weight 2 Weight 3 1 (0.5774, 0.0000) (0.5774,0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (−0.2887, 0.5000) (−0.2887,−0.5000) (0.5774, 0.0000) (−0.2887, −0.5000) (−0.2887, 0.5000) 2(0.5774, 0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (0.2887, 0.5000)(−0.5774, 0.0000) (0.2887, −0.5000) (−0.2887, 0.5000) (0.5774, 0.0000)(−0.2887, −0.5000)

TABLE 7 Number of Transmit Antennas [n_(T)]: 3, Number of Weight Sets[N]: 3 Set Weight 1 Weight 2 Weight 3 1 (0.5774, 0.0000) (0.5774,0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (−0.2887, 0.5000) (−0.2887,−0.5000) (0.5774, 0.0000) (−0.2887, −0.5000) (−0.2887, 0.5000) 2(0.5774, 0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (0.4423, 0.3711)(−0.5425, 0.1975) (0.1003, −0.5686) (0.1003, 0.5686) (0.4423, −0.3711)(−0.5425, −0.1975) 3 (0.5774, 0.0000) (0.5774, 0.0000) (0.5774, 0.0000)(0.1003, 0.5686) (−0.5774, −0.1975) (0.4423, −0.3711) (−0.5425, 0.1975)(0.4423, 0.3711) (0.1003, −0.5686)

TABLE 8 Number of Transmit Antennas [n_(T)]: 3, Number of Weight Sets[N]: 4 Set Weight 1 Weight 2 Weight 3 1 (0.5774, 0.0000) (0.5774,0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (−0.2887, 0.5000) (−0.2887,−0.5000) (0.5774, 0.0000) (−0.2887, −0.5000) (−0.2887, 0.5000) 2(0.5774, 0.0000) (0.5774, 0.0000) (0.5774, 0.0000) (0.5000, 0.2887)(−0.5000 0.2887) (0.0000, −0.5774) (0.2887, 0.5000) (0.2887, −0.5000)(−0.5774, 0.0000) 3 (0.5774, 0.0000) (0.5774, 0.0000) (0.5774, 0.0000)(0.2887, 0.5000) (−0.5774, 0.0000) (0.2887, −0.5000) (−0.2887, 0.5000)(0.5774, 0.0000) (−0.2887, −0.5000) 4 (0.5774, 0.0000) (0.5774, 0.0000)(0.5774, 0.0000) (0.0000, 0.5774) (−0.5000, −0.2887) (0.5000, −0.2887)(−0.5774, 0.0000) (0.2887, 0.5000) (0.2887, −0.5000)

TABLE 9 Number of Transmit Antennas [n_(T)]: 4, Number of Weight Sets[N]: 1 Set Weight 1 Weight 2 Weight 3 Weight 4 1 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000)(0.0000, 0.5000) (−0.5000, 0.0000) (0.0000, −0.5000) (0.5000, 0.0000)(−0.5000, 0.0000) (0.5000, 0.0000) (−0.5000, 0.0000) (0.5000, 0.0000)(0.0000, −0.5000) (−0.5000, 0.0000) (0.0000, 0.5000)

TABLE 10 Number of Transmit Antennas [n_(T)]: 4, Number of Weight Sets[N]: 2 Set Weight 1 Weight 2 Weight 3 Weight 4 1 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000)(0.0000, 0.5000) (−0.5000, 0.0000) (0.0000, −0.5000) (0.5000, 0.0000)(−0.5000, 0.0000) (0.5000, 0.0000) (−0.5000, 0.0000) (0.5000, 0.0000)(0.0000, −0.5000) (−0.5000, 0.0000) (0.0000, 0.5000) 2 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.3536, 0.3536)(−0.3536, 0.3536) (−0.3536, −0.3536) (0.3536, −0.3536) (0.5000, 0.5000)(0.0000, −0.5000) (0.0000, 0.5000) (0.0000, −0.5000) (−0.3536, 0.3536)(0.3536, 0.3536) (0.3536, −0.3536) (−0.3536, −0.3536)

TABLE 11 Number of Transmit Antennas [n_(T)]: 4, Number of Weight Sets[N]: 3 Set Weight 1 Weight 2 Weight 3 Weight 4 1 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000)(0.0000, 0.5000) (−0.5000, 0.0000) (0.0000, −0.5000) (0.5000, 0.0000)(−0.5000, 0.0000) (0.5000, 0.0000) (−0.5000, 0.0000) (0.5000, 0.0000)(0.0000, −0.5000) (−0.5000, 0.0000) (0.0000, 0.5000) 2 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.4330, 0.2500)(−0.2500, 0.4330) (−0.4330, −0.2500) (0.2500, −0.4330) (0.2500, 0.4330)(−0.2500, −0.4330) (0.2500, 0.4330) (−0.2500, −0.4330) (0.0000, 0.5000)(0.5000, 0.0000) (0.0000, −0.5000) (−0.5000, 0.0000) 3 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.2500, 0.4330)(−0.4330, 0.2500) (−0.2500, −0.4330) (0.4330, −0.2500) (−0.2500, 0.4330)(0.2500, −0.4330) (−0.2500, 0.4330) (0.2500, −0.4330) (−0.5000, 0.0000)(0.0000, 0.5000) (0.5000, 0.0000) (0.0000, −0.5000)

TABLE 12 Number of Transmit Antennas [n_(T)]: 4, Number of Weight Sets[N]: 4 Set Weight 1 Weight 2 Weight 3 Weight 4 1 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000)(0.0000, 0.5000) (−0.5000, 0.0000) (0.0000, −0.5000) (0.5000, 0.0000)(−0.5000, 0.0000) (0.5000, 0.0000) (−0.5000, 0.0000) (0.5000, 0.0000)(0.0000, −0.5000) (−0.5000, 0.0000) (0.0000, 0.5000) 2 (0.5000, 0.0000)(0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.4619, 0.1913)(−0.1913, 0.4619) (−0.4619, −0.1913) (0.1913, −0.4619) (0.3536, 0.3536)(−0.3536, −0.3536) (0.3536, 0.3536) (−0.3536, −0.3536) (0.1913, 0.4619)(0.4619, −0.1913) (−0.1913, −0.4619) (−0.4619, 0.1913) 3 (0.5000,0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.3536,0.3536) (−0.3536, 0.3536) (−0.3536, −0.3536) (0.3536, −0.3536) (0.0000,0.5000) (0.0000, −0.5000) (0.0000, 0.5000) (0.0000, −0.5000) (−0.3536,0.3536) (0.3536, 0.3536) (0.3536, −0.3536) (−0.3536, −0.3536) 4 (0.5000,0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.5000, 0.0000) (0.1913,0.4619) (−0.4619, 0.1913) (−0.1913, −0.4619) (0.4619, −0.1913) (−0.3536,0.3536) (0.3536, −0.3536) (−0.3536, 0.3536) (0.3536, −0.3536) (−0.4619,−0.1913) (−0.1913, 0.4619) (0.4619, 0.1913) (0.1913, −0.4619)

2. Second Exemplary Embodiment

A scheme for feeding back weight selection information using a feedbackchannel for state information of sub-data streams.

Information for indicating weights selected from one weight set can befed back in the following two schemes.

The first scheme uses a dedicated feedback channel for transferring onlyinformation about weights selected from one weight set as in the firstexemplary embodiment of the present invention. This scheme uses 1-bitfeedback information for each weight to indicate selection ornon-selection of each weight belonging to the selected weight set. Anamount of precode feedback information is a total of └log₂N┘+n_(T)bits/use including feedback information for transmitting an index of theselected weight set. To adjust transmission data rates of respectivedata streams to be transmitted, channel state information of therespective data streams formed by the selected weights is to beadditionally fed back. When the dedicated feedback channel is used totransfer weight selection information, channel state information ofsub-data streams relative to unselected weights does not need to be fedback.

The second scheme is a feedback scheme for transferring weight selectioninformation in accordance with the second exemplary embodiment of thepresent invention. To adjust transmission data rates of respective datastreams to be transmitted, the scheme uses a feedback channel forchannel state information of the respective data streams in a system forfeeding back channel state information of respective sub-data streamsfrom the receiver to the transmitter.

FIG. 6 illustrates a knockdown precoding system 600 in accordance withthe second exemplary embodiment of the present invention. The samecomponents between the first and second exemplary embodiments aredenoted by the same reference numerals. Only differences between thefirst and second exemplary embodiments will be described, but parts forperforming the same functions are omitted or will be briefly described.

The second exemplary embodiment will be briefly described with referenceto FIG. 6. A weight selector 631 of the system 600 of the exemplaryembodiment selects weights relative to the maximum data transmissionrate using fading channel information estimated in the downlink channelestimator 133 and transfers weight vectors selected from an associatedweight set to a subchannel-by-subchannel state estimator 623. Thesubchannel-by-subchannel state estimator 623 computes and quantizesSINRs of sub-data streams formed by the weight selector 631, and decideschannel state information 653 of sub-data streams, in other words aChannel Quality Indicator (CQI) or MCS. Herein, a combination of variousmodulation schemes and coding rates is possible in the channel stateinformation 653 of the sub-data streams. This example can be shown inTable 13.

TABLE 13 Coding Rate Modulation Scheme ½ QPSK 8PSK 16QAM 64QAM ¾ QPSK8PSK 16QAM 64QAM

To feed back information about weights selected from one weight set inthis exemplary embodiment, a “No Transmission” level is added toindicate that an associated weight has not been used in the existing MCSor CQI level. That is, when an associated weight has not been selectedfor transmission, the “No Transmission” level is fed back through thestate information 653 of the respective sub-data streams.

A controller 611 of the transmitter 610 receives feedback information650 including weight set index information 651 and state information 653of respective sub-data streams. The controller 611 decides the number ofsub-data streams capable of being simultaneously transmitted using thestate information 653 of the respective data streams and notifies ademultiplexer 113 of the decided number. Moreover, the controller 611decides a coding rate, a modulation scheme, and an associated weight ofeach sub-data stream using the feedback information 650, and notifieschannel encoders/modulators 115 and 117 and beamformers 119 and 121 ofdecision results.

An exemplary method for transmitting and receiver data in thetransmitter and the receiver of the system 600 of the present inventionwill be described in detail with reference to FIGS. 7 and 8.

FIG. 7 is a flowchart illustrating a method for transmitting/receivingdata in the receiver 630 of the system 600 in accordance with the secondexemplary embodiment of the present invention.

Referring to FIG. 7, a downlink channel estimator 133 of the receiver630 estimates a downlink fading channel using a pilot channel or symbolreceived from a plurality of receive antennas in step 701. That is, thedownlink fading channel from each transmit antenna to each receiveantenna is estimated. Subsequently, the weight selector 631 decides aweight set relative to a maximum data transmission rate and weightvectors selected from an associated weight set in step 703. Thesubchannel-by-subchannel state estimator 623 is notified of decisioninformation. The subchannel-by-subchannel state estimator 623 estimateschannel states of the respective sub-data streams according to thedecided weight vectors in step 705. That is, thesubchannel-by-subchannel state estimator 623 computes SINRs of sub-datastreams formed by the selected weights, and decides channel stateinformation 653 of the respective sub-data streams. In step 707, thereceiver 630 transmits feedback information 650 including a weight setindex 651, transmission information, and the channel state informationof the respective sub-data streams.

Next, step 707 will be described in detail. State information of weightsunselected in step 703 is set to a “No Transmission” level, and stateinformation of all weights belonging to a selected weight set is fedback to the transmitter 610. Accordingly, only a feedback channel fortransferring the selected weight set index information 651 and afeedback channel for transferring the channel state information 653 ofthe respective sub-data streams formed by estimated selected weights arerequired. Herein, the weight set index 651 and the state information 653of the respective sub-data streams can be simultaneously or separatelytransmitted.

FIG. 8 is a flowchart illustrating a method for transmitting/receivingdata in the transmitter 610 of the system 600 in accordance with thesecond exemplary embodiment of the present invention.

Referring to FIG. 8, the transmitter 610 receives feedback information650 from the receiver 630 in step 801. Subsequently, the controller 611sets the number of sub-data streams capable of being finally transmittedusing channel state information 653 of the respective sub-data streamsin step 803. Because a channel to be unused is set to a “NoTransmission” level, the controller 611 can know the channel stateinformation 653 of the respective sub-data streams. The demultiplexer113 demultiplexes a main data stream to be transmitted into sub-datastreams capable of being transmitted in step 805. Using a coding rateand a modulation scheme, the channel encoders/modulators 115 and 117independently performs encoding and symbol mapping processes for thesub-data streams in step 807. Subsequently, the beamformers 119 and 121multiply the sub-data streams by weights, perform a beamforming process,and transmit the encoded and modulated sub-data streams to the receiver630 in step 809. The transmitter 610 can know the weights becauseselection information of the weights belonging to an associated weightset is included in the channel state information 653 of the respectivesub-data streams.

Because MCS information relative to weights unused for an actualtransmission must be also fed back, a total amount of precode and MCSfeedback information is considered. The second exemplary embodimentrequires a smaller amount of feedback information than the firstexemplary embodiment only when the number of weights to be used for theactual transmission is less than ((½ of Total Number of TransmitAntennas)+1). When the number of weights to be used for the actualtransmission is more than ((½ of Total Number of Transmit Antennas)+1),a feedback channel for feeding back the channel state information of therespective sub-data streams is used to transmit the feedbackinformation. In other cases, feedback in which two schemes are combinedcan be performed with a dedicated feedback channel for transferring onlyweight selection information.

3. Third Exemplary Embodiment

Open-loop Knockdown Precoding Technology

The proposed knockdown precoding technology can operate as open-loopknockdown precoding technology and closed-loop knockdown precodingtechnology according to the presence of a feedback channel fortransferring selected weight set index information.

The closed-loop knockdown precoding technology in the above-describedfirst and second exemplary embodiments corresponds to the case whereindex information of a selected common weight set is fed back. Accordingto a feedback scheme for transferring weight selection information, thestructures and operations of the transmitter and the receiver of theopen-loop knockdown precoding system in the third exemplary embodimentare the same as those of the knockdown precoding system 100 using thededicated feedback channel of FIG. 1 or the knockdown precoding system600 using a feedback channel for feeding back thesubchannel-by-subchannel state information in FIG. 6.

In the third exemplary embodiment, a feedback channel for transferringan index of a selected weight set is absent but a feedback channel fortransferring information about selected weights is present. When thenumber of transmit antennas is two, the knockdown precoding systemstructure of the open-loop scheme is the same as those of FIGS. 1 and 6.

A system 900 in accordance with the third exemplary embodiment of thepresent invention will be described with reference to FIG. 9. In thethird exemplary embodiment, a description of the same parts as those ofthe first and second exemplary embodiments is omitted or will be brieflygiven.

Referring to FIG. 9, the system 900 of the third exemplary embodiment isalmost the same as that of the above-described closed-loop knockdownprecoding technology. A difference is that a feedback channel for aselected common weight set index is absent in the system 900. Becausethe feedback channel for the selected common weight set index is absent,a transmitter 910 and receivers 930 within a cell use only one weightset in one time slot. A weight set to be used is not fixed, and N weightsets are sequentially and periodically used. That is, the weight setsmay be used in order of E₁, E₂, E₃, . . . , E_(N), E_(l). A weight setto be used varies with a defined period and order. Accordingly, thecontroller 911 controls demultiplexing, encoding, modulation, andbeamforming processes for a main data stream using selected weightvector information 951 and state information 953 of respective sub-datastreams. Because a data processing method of the transmitter 910 is thesame as those of the above-described exemplary embodiments, itsdescription is omitted herein.

When a weight set in a predetermined time slot is known, the weightselector 931 selects weight vectors from the weight set and feeds backweight vector information 951. The subchannel-by-subchannel stateestimator 933 detects the selected weight vectors from the weightselector 931 and notifies the transmitter 910 of the state information953 of the sub-data streams.

A method for transmitting/receiving data in the system 900 of the thirdexemplary embodiment will be described with reference to FIGS. 10 and11.

FIG. 10 is a flowchart illustrating a method for transmitting/receivingdata in a receiver 930 of the system 900 in accordance with the thirdexemplary embodiment of the present invention.

Referring to FIG. 10, a downlink channel estimator 133 estimates afading channel from each transmit antenna to each receive antenna usinga pilot channel or symbol received from a plurality of receive antennas139 in step 1001. Subsequently, the weight selector 931 selects weightvector information 951 to be actually transmitted because a weight setis known in a time period in step 1003. The subchannel-by-subchannelstate estimator 933 estimates channel states of respective sub-datastreams according to the selected weight vectors in step 1005.Subsequently, the receiver 930 transmits feedback information 950including the weight vector information 951 and channel stateinformation 953 of the respective sub-data streams to the transmitter910 in step 1007.

FIG. 11 is a flowchart illustrating a method for transmitting/receivingdata in the transmitter 910 of the system 900 in accordance with thethird exemplary embodiment of the present invention.

Referring to FIG. 11, the transmitter 910 decides the number of sub-datastreams capable of being simultaneously transmitted using feedbackinformation 950 in step 1103 when receiving the feedback information 950in step 1101. Then, the demultiplexer 113 demultiplexes a main datastream to be transmitted into sub-data streams capable of beingtransmitted in step 1105. Using a coding rate and a modulation scheme,the channel encoders/modulators 115 and 117 independently performencoding and symbol mapping processes for the sub-data streams in step1107. Subsequently, the beamformers 119 and 121 multiply the sub-datastreams by weights, perform a beamforming process, and transmit theencoded and modulated sub-data streams to the receiver in step 1109.

In the open-loop scheme of the system 900 of the third exemplaryembodiment, a channel for feeding back a selected weight set index isabsent and one weight set is only used in one time slot. A transmissiondata rate of the open-loop scheme is lower than the closed-loop schemeof the first and second exemplary embodiments. However, because anamount of feedback information of the open-loop scheme is less than thatof the closed-loop scheme, the open-loop scheme is applied to a systemin which an amount of feedback information to be transmitted is limited,such that a transmission data rate in the preceding scheme is improved.

Comparison and analysis between the proposed technology and theconventional technology

The conventional precoder codebook technology and the proposed knockdownprecoding technology are compared and analyzed in terms of a scheme foradjusting the number of data streams to be simultaneously transmittedand an amount of feedback information required therefor.

In the conventional precoder codebook technology, a precoder codebook isseparately defined and used according to the number of transmitantennas, n_(T), the number of receive antennas, n_(R), and the numberof data streams to be simultaneously transmitted, n_(S). When the numberof data streams to be simultaneously transmitted is adjusted accordingto a channel state of each transmitter/receiver in a state in which thetransmitter with four transmit antennas communicates with receivers inwhich the number of receive antennas is one, two, three, and four, thenumber of precoder codebooks to be considered is 10, in other words(n_(T), n_(R), n_(S))=(4, 1, 1), =(4,1,1), (4,2,1), (4,2,2), (4,3,1),(4,3,2), (4,3,3), (4,4,1), (4,4,2), (4,4,3), and (4,4,4). The 10precoder-codebooks are defined between the transmitter and thereceivers. The receiver feeds back the number of receive antennas,n_(R), and the number of data streams, n_(S), to the transmitter, suchthat the transmitter selects a precoder codebook to be used. Thereceiver selects a precode for a transmission at the maximum capacity ina precoder codebook suitable for the number of receive antennas, n_(R),and the number of data streams, n_(S), and feeds back an index of theselected precode to the transmitter. The transmitter selects a precodewith the feedback index in the precoder codebook suitable for thefed-back n_(R) and n_(S), and transmits data.

Because n_(R) can be fed back only once, an amount of required feedbackinformation for n_(R) is small and negligible. Feedback information forn_(S) instantaneously varying with a channel state is to be transmittedalong with feedback information of an index of a selected precode.Assuming that each precoder codebook is constructed by 8 precodes, 2bits/use for feedback information of n_(S) and 3 bits/use for feedbackinformation of the selected precode index are required, such that atotal of 5 bits/use for feedback information are required.

An optimal precoder codebook differs according to a spatial correlationof fading in an operating channel. Up to now, a precoder codebook hasbeen designed under the assumption that a spatial correlation of fadingis absent in the conventional precoder codebook technology. Accordingly,performance degradation occurs in a channel environment in which aspatial correlation of fading is present. To overcome the performancedegradation, the transmitter is to perform a companding process for anexisting precoder codebook using a spatial correlation matrix of adownlink channel. For this, because the receiver estimates the spatialcorrelation matrix of the downlink channel and feeds back the estimatedmatrix to the transmitter, an additional amount of feedback informationfor feeding back a spatial correlation matrix of a downlink channel aswell as an amount of feedback information for feeding back n_(S) and aselected index is required.

In the knockdown preceding technology proposed in the present invention,N weight sets constructed by orthogonal weights whose number correspondsto the number of transmit antennas, n_(T), are defined. The receiverconsiders the number of used receive antennas, n_(R), and selects amaximum of min (n_(T),n_(R)) weights relative to the maximumtransmission rate. The receiver feeds back the selected weights to thetransmitter through feedback information of a selected weight set indexand weights selected from the associated set. The transmitter transmitsmultiple data streams using weights selected from the weight set basedon the feedback information. Because N weight sets configured by a totalof N.n_(T) weights are commonly used even though receive antennas ofreceivers are various and the number of data streams to besimultaneously transmitted is various, an amount of information aboutweight sets to be defined between the transmitter and the receivers issignificantly smaller than an amount of information required in theprecoder codebook scheme. Specifically, because the number of precodercodebooks to be considered significantly increases when the number oftransmit antennas exceeds four, an amount of information about theprecoder codebooks to be defined between the transmitter and thereceivers significantly increases. In contrast, in the proposedknockdown precoding scheme, an amount of information about weight setsto be defined between the transmitter and the receivers almost does notincrease because the number of weight sets, N, decreases even when thenumber of transmit antennas, n_(T), increases. This is because theperformance of the knockdown precoding technology depends on the numberof weights, N.n_(T).

An amount of feedback information required in the closed-loop knockdownprecoding technology using a dedicated feedback channel for weightselection information feedback is └log₂┘ bits/use for feeding back aselected weight set index and n_(T) bits/use for feeding back weightselection information, such that a total of └log₂┘+n_(T) bits/use arerequired. When the number of transmit antennas is 4 and N=2, a total of5 bits/use are required. An amount of feedback information required inthe open-loop knockdown precoding technology using a dedicated feedbackchannel for feeding back weight selection information is only n_(T)bits/use for feeding back weight selection information. To reduce anamount of feedback information required for weight selectioninformation, a scheme for feeding back weight selection informationusing a feedback channel for transmitting channel state information ofrespective sub-data streams can be used.

A feedback scheme can be selected to transmit weight selectioninformation according to an uplink channel structure of a system towhich the proposed knockdown precoding technology is applied. The numberof weight sets to be used can be adjusted and applied according touplink channel capacity available in the system. Specially, when theuplink channel capacity available in the system is very small, the openknockdown precoding technology can be applied.

FIG. 12 illustrates performance comparison results of a Minimum MeanSquare Error-Ordered Successive Interference Cancellation (MMSE-OSIC)system using the proposed knockdown precoding technology and theprecoder codebook technology in an environment in which a spatialcorrelation is high when n_(T)=n_(R)=4. When the knockdown precodingtechnology considers the case where two weight sets are used, theclosed-loop knockdown precoding technology requires one bit for feedbackof a weight set index and four bits for feedback of selectioninformation of four weights, in other words a total of 5 bits/use. Theopen-loop knockdown precoding technology requires 4 bits/use forfeedback of selection information of four weights. The precoder codebooktechnology requires 2 bits/use for adjusting the number of data streamsto be simultaneously transmitted and 3 bits/use for feedback of aselected precode index, in other words an amount of feedback informationof a total of 5 bits/use. When the performances of the closed-loopknockdown techoology and the precoder codebook technology withoutcompanding requiring the same amount of feedback information arecompared, it can be seen that the closed-loop knockdown precodingtechnology outperforms the precoder codebook technology withoutcompanding. In addition, it can be seen that the open-loop knockdownprecoding technology requiring 4 bits/use outperforms the precodercodebook technology requiring 5 bits/use without companding. Theprecoder codebook technology with companding has a performance similarto that of the closed-loop knockdown precoding technology. Becauseadditional feedback for a spatial correlation matrix of a downlinkchannel for companding is required, its amount of required feedbackinformation is significantly larger than that of the closed-loopknockdown precoding technology.

From the simulation results, it can be seen that the proposed knockdownprecoding technology is more easily applied to a channel environmentwith various spatial correlations and has more excellent performance incomparison with the conventional precoder codebook technology.

FIG. 13 illustrates performance comparison results of a MMSE-OSIC systemusing the proposed knockdown precoding technology and the precodercodebook technology in an environment in which a spatial correlation isabsent when n_(T)=n_(R)=4.

Referring to FIG. 13, the precoder codebook technology with compandinghas the same performance as the precoder codebook technology withoutcompanding in an uncorrelated environment, because a transmissioncorrelation matrix is an identity matrix in the uncorrelated environmentand a precoder codebook is not varied even though companding isperformed. The two precoder-codebook technologies have the sameperformance as the closed-loop knockdown precoding technology andslightly outperform the open-loop knockdown preceding technology. Fromthe performance results of FIGS. 12 and 13, it can be seen that theproposed precoder codebook technology has performance similar to that ofthe conventional technology in the uncorrelated environment andoutperforms the conventional technology in a channel environment withvarious spatial correlations.

As described above, the knockdown precoding technology of the presentinvention can be more easily applied to a channel environment withvarious spatial correlations and can have more excellent performance andhigher throughput in comparison with the conventional precoder codebooktechnology. The knockdown precoding technology requires a smaller memorysize than the precoder codebook technology, and can be optimizedaccording to an uplink channel structure and capacity of a system towhich the spatial multiplexing technology is applied.

Although the exemplary embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions, and substitutions arepossible, without departing from the scope of the present invention. Inthe example of the present invention, the system in which the number oftransmit antennas and the number of receive antennas are two has beendescribed for convenience of explanation. Of course, at least threeantennas can be applied. Therefore, the present invention is not limitedto the above-described embodiments, but is defined by the followingclaims, along with their full scope of equivalents.

What is claimed is:
 1. A mobile communication system using multipleantennas, comprising: a receiver for estimating a fading channel ofreceived data, selecting a weight set from a plurality of weight sets ona basis of the estimated fading channel, and transmitting feedbackinformation including information indicating the selected weight set andchannel-by-channel state information to a transmitter, wherein eachweight set has elements of a plurality of orthogonal weight vectorscorresponding to weights used in the multiple antennas, with theplurality of orthogonal weight vectors of each weight set havingdifferent phases between each other; and the transmitter fordemultiplexing data to be transmitted on a basis of the feedbackinformation into at least one sub-data stream, multiplying each sub-datastream by an associated weight, and transmitting the data.
 2. The mobilecommunication system of claim 1, wherein the receiver comprises: adownlink channel estimator for estimating a channel state using a pilotchannel of the data transmitted from the transmitter; a weight selectorfor deciding the weight set and weight vectors on a basis of the channelstate, and transmitting information about the weight set and informationindicating the decided weight vectors to the transmitter; and asubchannel-by-subchannel state estimator for estimating channel statesof sub-data streams according to the decided weight vectors andtransmitting only information about the channel states of the sub-datastreams to the transmitter.
 3. The mobile communication system of claim1, wherein the receiver comprises: a downlink channel estimator forestimating a channel state using a pilot channel of the data transmittedfrom the transmitter; a weight selector for deciding the weight set andweight vectors on a basis of the channel state, and transmittinginformation indicating the decided weight set and the decided weightvectors to the transmitter; and a subchannel-by-subchannel stateestimator for estimating channel states of all weight vectors of thedecided weight set and transmitting information about the estimatedchannel states to the transmitter.
 4. The mobile communication system ofclaim 2 or 3, wherein the transmitter comprises: a demultiplexer fordemultiplexing a main data stream to be transmitted into the at leastone sub-data stream; at least one channel encoder and modulator forreceiving the at least one sub-data stream and independently performingchannel coding and modulation processes for the at least one sub-datastream according to a predefined channel coding rate and a predefinedmodulation scheme; at least one beamformer for multiplying the at leastone channel-coded and modulated sub-data stream by a predefined weightand transmitting the data to the receiver; and a controller for decidingin advance the number of sub-data streams, the coding rate of the atleast one sub-data stream, the modulation scheme, and a weight to bemultiplied by each sub-data stream on a basis of the feedbackinformation transmitted from the receiver.
 5. The mobile communicationsystem of claim 3, wherein the subchannel-by-subchannel state estimatortransmits information about a “No Transmission” state for an unusedchannel on a basis of the decided weight vectors.
 6. The mobilecommunication system of claim 1, wherein the feedback informationcomprises: weight set index information for indicating the selectedweight set; weight vector information for indicating weight vectorsselected from the selected weight set; and channel state information ofthe at least one sub-data stream.
 7. The mobile communication system ofclaim 1, wherein the transmitter and the receiver store weight sets andweight vectors according to the number of transmit antennas and thenumber of weight sets.
 8. A method for transmitting/receiving data in amobile communication system using multiple antennas, the methodcomprising: a) estimating a fading channel from a pilot channel ofreceived data in a receiver; b) selecting a weight set from a pluralityof weight sets on a basis of the estimated fading channel, wherein eachweight set has elements of a plurality of orthogonal weight vectorscorresponding to weights used in the multiple antennas, with theplurality of orthogonal weight vectors of each weight set havingdifferent phases between each other; c) estimating channel-by-channelstate information relative to the selected weight set; d) transmittingfeedback information comprising information indicating the selectedweight set and the channel-by-channel state information to atransmitter; and e) transmitting, by the transmitter, antenna-by-antennadata on a basis of the feedback information.
 9. The method of claim 8,wherein designing the weight set comprises: deciding a plurality ofweight vectors with a phase difference defined according to the numberof transmit antennas and the number of weight sets; and configuring theweight set with orthogonal weight vectors among the decided weightvectors.
 10. The method of claim 9, wherein the phase difference iscomputed by $\frac{2\pi}{N \cdot n_{T}},$ where N is the number ofweight sets and n_(T) is the number of transmit antennas.
 11. The methodof claim 8, wherein designing the weight set comprises: f-1) deciding aplurality of orthogonal weight vectors among a number of weight sets;and f-2) repeating f-1) a number of times corresponding to the number ofweight sets.
 12. The method of claim 11, wherein f-1) comprises:deciding a reference phase for orthogonal elements of a decided weightvector; and deciding elements with a difference of the reference phasefrom a first element of the weight vector.
 13. The method of claim 8,wherein the feedback information comprises: weight set index informationfor indicating the selected weight set; weight vector information forindicating weight vectors selected from the selected weight set; andchannel state information of at least one sub-data stream.
 14. Themethod of claim 8, wherein the feedback information comprises: weightset index information for indicating the selected weight set; andchannel state information of all sub-data streams relative to theselected weight set.
 15. The method of claim 8, wherein e) comprises:demultiplexing a main data stream to be transmitted into at least onesub-data stream on the basis of the feedback information; independentlyperforming channel coding and modulation processes for the at least onesub-data stream according to a channel coding rate and a modulationscheme defined on the basis of the feedback information; and multiplyingthe at least one channel-coded and modulated sub-data stream by a weightdefined on the basis of the feedback information and transmitting thedata to the receiver.
 16. A method for transmitting feedback informationfrom a receiver to a transmitter in a mobile communication system usingmultiple antennas, the method comprising: a) estimating a fading channelfrom a pilot channel of received data and selecting a weight set from aplurality of weight sets on a basis of the estimated fading channel,wherein the weight set has elements of a plurality of orthogonal weightvectors corresponding to weights used in the multiple antennas, with theplurality of orthogonal weight vectors of each weight set havingdifferent phases between each other; b) estimating channel-by-channelstate information according to the selected weight set; and c)transmitting, to the transmitter, feedback information comprising indexinformation of the selected weight set and the channel-by-channel stateinformation.
 17. The method of claim 16, wherein the feedbackinformation comprises: weight set index information for indicating theselected weight set; weight vector information for indicating weightvectors selected from the selected weight set; and channel stateinformation of at least one sub-data stream.
 18. The method of claim 16,wherein the feedback information comprises: weight set index informationfor indicating the selected weight set; and channel state information ofall sub-data streams relative to the selected weight set.
 19. A receiverin a mobile communication system using multiple antennas, comprising: adownlink channel estimator for estimating a channel state using a pilotchannel of the data transmitted from a transmitter; a weight selectorfor deciding a weight set and weight vectors on a basis of the channelstate and transmitting information about the weight set and informationindicating the weight vectors to the transmitter, wherein the weight sethas elements of a plurality of orthogonal weight vectors correspondingto weights used in the multiple antennas, with the plurality oforthogonal weight vectors of each weight sets having different phasesbetween each other; and a subchannel-by-subchannel state estimator forestimating channel states of sub-data streams according to the decidedweight vectors and transmitting only information about the channelstates of the sub-data streams to the transmitter.
 20. A method fordesigning a weight set to be used in a data transceiver of a mobilecommunication system comprising the transceiver with multiple antennas,the method comprising: deciding a plurality of weight vectors with aphase difference defined according to the number of transmit antennasand the number of weight sets, wherein the weight set has elements ofmultiple weight vectors corresponding to weights used in the multipleantennas; and configuring the weight set with orthogonal weight vectorsamong the decided weight vectors, wherein the phase difference iscomputed by $\frac{2\pi}{N \cdot n_{T}},$  where N is the number ofweight sets and n_(T) is the number of transmit antennas.
 21. The methodof claim 20, wherein designing the weight set comprises: a) deciding aplurality of orthogonal weight vectors among a number of weight sets;and b) repeating a) a number of times corresponding to the number ofweight sets.
 22. The method of claim 21, wherein a) comprises: decidinga reference phase for orthogonal elements of a decided weight vector;and deciding elements with a difference of the reference phase from afirst element of the weight vector.
 23. A mobile communication systemwith a transceiver using multiple antennas, comprising: a receiver forproviding a weight set decided by estimating a channel of received datato a transmitter; and the transmitter for demultiplexing data to betransmitted into sub-data streams on a basis of the provided weight setand transmitting the sub-data streams to the receiver; wherein theweight set has elements of multiple weight vectors corresponding toweights used in the multiple antennas, the weight vectors beingorthogonal to each other; wherein the weight vector is computed by:${e_{n,\; i} = {{\frac{1}{\sqrt{n_{T}}}\begin{bmatrix}\omega_{1,\; i}^{(n)} \\\vdots \\\omega_{n_{T},\; i}^{(n)}\end{bmatrix}} = {\frac{1}{\sqrt{n_{T}}}\begin{bmatrix}1 \\e^{j\;\frac{\;{2\;\pi}}{n_{T}}\;{({\frac{n - 1}{N} + {({i - 1})}})}} \\e^{j\; 2\;\frac{\;{2\;\pi}}{n_{T}}\;{({\frac{n - 1}{N} + {({i - 1})}})}} \\\vdots \\e^{{j{({n_{T}\; - \; 1})}}\;\frac{2\;\pi}{n_{T}}\;{({\frac{n - 1}{N} + {({i - 1})}})}}\end{bmatrix}}}},$ where e_(n,i) is an i-th weight vector belonging toan n-th weight set.
 24. A receiver in a mobile communication systemusing multiple antennas, comprising: a downlink channel estimator forestimating a fading channel using a pilot channel of the datatransmitted from a transmitter; and a weight selector for decidingfeedback information based on a weight set selected to be applied in atime period according to channel state, and for transmitting the decidedfeedback information to the transmitter; wherein the weight set isselected from among a plurality of predetermined weight sets accordingto a predetermined order corresponding to one of a plurality ofpredefined time periods.
 25. The receiver of claim 24, wherein thereceiver further comprises: a subchannel-by-subchannel state estimatorfor estimating channel states of weight vectors of the weight set andtransmitting information about the estimated channel states to thetransmitter.
 26. The receiver of claim 24, wherein the receiver storesin advance at least one weight set and weight vectors.
 27. A transmitterin a mobile communication system using multiple antennas, comprising: acontroller for selecting sequentially a weight set corresponding to eachof a plurality of predefined time periods from among a plurality ofweight sets in a codebook, receiving feedback information transmittedfrom a receiver and decided on the selected weight set by the receiverprior to transmission of the feedback information, and determining,based on the feedback information, a count in number of sub-data streamsthrough which a main data stream is to be transmitted to the receiver;and a beamformer for multiplying a number of channel-coded and modulatedsub-data streams, which have been formed to transmit the main datastream and whose count in number of sub-data streams is equal to thecount determined based on the feedback information, by weight vectors ofthe selected weight set, and transmitting the sub-data streams resultingfrom the multiplying operation to the receiver.
 28. The transmitter ofclaim 27, wherein the transmitter further comprises: a demultiplexer fordemultiplexing a main data stream to be transmitted into at least onesub-data stream; and at least one channel encoder and modulator forreceiving the at least one sub-data stream and independently performingchannel coding and modulation processes for the at least one sub-datastream according to a channel coding rate and a modulation scheme. 29.The transmitter of claim 27, wherein the transmitter store in advance atleast one weight set and weight vectors.
 30. A method for transmittingfeedback information by a receiver in a mobile communication systemusing multiple antennas, the method comprising: estimating a fadingchannel using a pilot channel of data transmitted from a transmitter;deciding feedback information based on a weight set selected to beapplied in a time period according to channel state; and transmittingthe decided feedback information to the transmitter; wherein the weightset is selected from among a plurality of predetermined weight setsaccording to a predetermined order corresponding to one of a pluralityof predetermined time periods.
 31. The method of claim 30, whereinchanging the weight set comprises: deciding a plurality of weightvectors with a phase difference defined according to the number oftransmit antennas and the number of weight sets; and configuring theweight set with orthogonal weight vectors among the decided weightvectors.
 32. The method of claim 31, wherein the phase difference iscomputed by $\frac{2\pi}{N \cdot n_{T}},$ where N is the number ofweight sets and n_(T) is the number of transmit antennas.
 33. The methodof claim 30, wherein designing the weight set comprises: a) deciding aplurality of orthogonal weight vectors among a number of weight sets;and b) repeating a) a number of times corresponding to the number ofweight sets.
 34. The method of claim 33, wherein a) comprises: decidinga reference phase for orthogonal elements of a decided weight vector;and deciding elements with a difference of the reference phase from afirst element of the weight vector.
 35. The method of claim 30, whereinthe receiver store in advance at least one weight set and weightvectors.
 36. A method for transmitting data by a transmitter in a mobilecommunication system using multiple antennas, the method comprising:selecting sequentially a weight set corresponding to each of a pluralityof predefined time periods from among a plurality of weight sets in acodebook; receiving feedback information transmitted from a receiver anddecided on the selected weight set by the receiver prior to transmissionof the feedback information, and determining, based on the feedbackinformation, a number in count of sub-data streams through which a maindata stream is to be transmitted to the receiver: and multiplying anumber of channel-coded and modulated sub-data streams, which have beenformed to transmit the main data stream and whose count in number ofsub-data streams is equal to the count determined based on the feedbackinformation, by weight vectors of the selected weight set, andtransmitting the number of sub-data streams resulting from themultiplying operation to the receiver.
 37. The method of claim 36,wherein the transmitter store in advance at least one weight set andweight vectors.
 38. A method for a terminal in wireless communication,the method comprising: selecting a precoding matrix from a codebookcomprising a plurality of precoding matrices; and transmittinginformation associated with the selected precoding matrix, wherein theplurality of precoding matrices are based on $\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\quad$  and $\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}\quad$  if the number of data streams to be transmittedthrough two antennas is equal to 2, and wherein j represents the squareroot of −1.
 39. The method of claim 38, wherein the precoding matrix isselected from a plurality of precoding matrices, and wherein theplurality of precoding matrices are based on${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},\quad$ ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}}\quad$  and ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}}\quad$  respectively if the number of data streams to betransmitted through the two antennas is equal to
 1. 40. The method ofclaim 38, wherein weight vectors in the codebook are orthogonal to eachother.
 41. A method for a base station in wireless communication, themethod comprising: selecting a precoding matrix from a codebookcomprising a plurality of precoding matrices, precoding data using theselected precoding matrix, and transmitting the precoded data, whereinthe plurality of precoding matrices are based on $\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\quad$  and $\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}\quad$  if the number of data streams to be transmittedthrough two antennas is equal to 2, and wherein j represents the squareroot of −1.
 42. The method of claim 41, wherein the precoding matrix isselected from a plurality of precoding matrices, and wherein theplurality of precoding matrices are based on${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},\quad$ ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}}\quad$  and ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}}\quad$  respectively if the number of data streams to betransmitted through the two antennas is equal to
 1. 43. The method ofclaim 41, wherein weight vectors in the codebook are orthogonal to eachother.
 44. A terminal in wireless communication, the terminalcomprising: a controller configured to select a precoding matrix from acodebook comprising a plurality of precoding matrices; and a transmitterconfigured to transmit information associated with the selectedprecoding matrix, wherein the plurality of precoding matrices are basedon $\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\quad$  and $\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}\quad$  if the number of data streams to be transmittedthrough two antennas is equal to 2, and wherein j represents the squareroot of −1.
 45. The terminal of claim 44, wherein the precoding matrixis selected from a plurality of precoding matrices, and wherein theplurality of precoding matrices are based on${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},\quad$ ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}}\quad$  and ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}}\quad$  respectively if the number of data streams to betransmitted through the two antennas is
 1. 46. The terminal of claim 44,wherein weight vectors in the codebook are orthogonal to each other. 47.A base station in wireless communication, the base station comprising: acontroller configured to: select a precoding matrix from a codebookcomprising a plurality of precoding matrices, and precode data using theselected precoding matrix; and a transmitter configured to transmit theprecoded data, wherein the plurality of precoding matrices are based on$\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\quad$  and $\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix},\quad$  if the number of data streams to be transmittedthrough two antennas is equal to 2, and wherein j represents the squareroot of −1.
 48. The base station of claim 47, wherein the precodingmatrix is selected from a plurality of precoding matrices, and whereinthe plurality of precoding matrices are based on${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}}\quad}$  and ${\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}}\quad$  respectively if the number of data streams to betransmitted through the two antennas is equal to
 2. 49. The base stationof claim 47, wherein weight vectors in the codebook are orthogonal toeach other.